Etabs 'STURM-SEQUENCE CHECK FOUND 2 EIGENVALUE(S) BELOW THE SHIFT'

[Catherine S]

My model cannot run the modal-ritz load case, the waring massage shows
THE STURM-SEQUENCE CHECK FOUND 2 EIGENVALUE(S) BELOW THE SHIFT, RITZ ANALYSIS REQUIRES ALL EIGENVALUES TO BE ABOVE THE SHIFT
and
NO RITZ MODE WERE FOUND

Do anyone know what causes this warning and how to solve it?

 

[JoshPlumSE]

Interesting. Sturm Sequence checks, as I understand them, are meant to check that the eigensolution found the lowest modes. I didn't realize that such a thing was possible with Ritz Vectors.

I suspect that the load used as the basis for your Ritz Vector is not very good. Or, your model is behaving strangely. If I were you, I would do the following:
a) Solve the model for regular loads and not dynamic ones to see if the model has any instabilities or near instabilities that need to be fixed.
b) Solve dynamically using a regular Eigen-Solution rather than a Ritz vector. And, look to see if there are any modes that indicate a problem with your model.
c) Take a closer look at the Ritz Vector.